<!DOCTYPE html>
<html>
<head>
	<title>undefined</title>
</head>
<body>
	<h2>使用undefined对变量进行清空</h2>
	<h2>空值和undefined不一样</h2>
	<p id="demo"></p>
	<script>
		var a = 3;
		a = undefined;
		document.getElementById('demo').innerHTML = 
		typeof a;
	</script>

</body>
</html>